With the constant increase of mobile data services and emergence of new-type applications such as multimedia online game (MMOG), mobile TV, Web2.0, and stream media, the 3rd Generation Partnership Project (3GPP) organization has developed long-term evolution (LTE) specifications. 3GPP LTE, which is known as an evolution standard of the Global System for Mobile Communications/High Speed Packet Access (GSM/HSPA) technology that has achieved a great success, aims at creating a new series of specifications for the new evolving radio-access technology, so as to go on improving the cellular communication system performance, such as achieving a higher throughput and a lower packet transmission latency.
An LTE system can operate both in Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode. In the FDD mode, the uplink and downlink employ a pair of frequency spectrums for data transmission; while in the TDD mode, the uplink and downlink channels share the same frequency, but occupy different time slots. Therefore, the TDD system has channel reciprocity, by which the downlink wireless channel information could be obtained with the knowledge get from the uplink channel.
FIG. 1A illustrates a typical scenario of a multi-cell LTE/LTE-A/TD-LTE system. As illustrated in FIG. 1A, the LTE system comprises a core network (EPC), base station devices (eNB1 to eNB3) and a plurality of user equipments UE1, UE2, . . . , UEn, . . . , UEm. It should be noted that although the shown system comprises three cells, it can be more in actual application. User equipment UE transmits data to eNB or receives data therefrom via their wireless interfaces; and respective eNB1 to eNB3 are further connected to the core network EPC.
In a downlink operation of the TDD system, a user equipment (UE) is responsible for measuring the downlink channel and feeding information back to a base station device (eNB) for using by the eNB to perform scheduling and allocating operations. FIG. 1B schematically illustrates a block diagram of a typical single-cell communication system according to the prior art, wherein the system comprises a plurality of UEs that communicate with an eNB. As illustrated in FIG. 1B, to enable the UE to fully appreciate the downlink channel, eNB transmits a cell specific reference signal (CRS) to the UE in some certain time and frequency combination resources (also called resource element RE) in the LTE system. The CRS is a pre-defined signal, pre-known to both the transmitter and the receiver; therefore, the UE can derive the downlink channel condition based on the received CRS. The CRS is not pre-coded and is transmitted over the entire system bandwidth of a cell. A data receiving unit 101 in the UE is for receiving CRS/data. A feedback calculation unit 102 is for measuring a feedback parameter, for example estimating a channel quality indication (CQI) based on the CRS. The feedback calculation unit 102 in the UE can calculate the CQI based on the channel information on some valid sub-frames, so as to obtain the CQI based on a PDSCH transmission scheme configured by a transmission mode (TM). For example, for mode 7 and mode 8 (hereinafter referred to as TM7 and TM8, respectively), if the number of PBCH antenna ports is one, then a single port solution is adopted; while if the number of PBCH antenna ports is more than one, then transmit diversity is adopted. A feedback transmission unit 103 is for transmitting to the eNB feedback information such as CQI, Pre-coding Matrix Information (PMI), Rank Indication (RI), etc. At the eNB, a scheduler unit 111 performs resource scheduling to each UE based on feedback information from the UE. Then, an allocation processing unit 112 performs channel resource allocation processing.
Besides, thanks to the channel reciprocity feature in the TDD system, it is possible for LTE to enable better performance for radio resource control and advanced antenna techniques. For example, in coverage limited areas like rural area, beamforming is one of the most effective ways to provide coverage extension and to reduce the number of cell sites. The enhanced signal strength to noise ratio allows more margins for UE decoding the data symbols, and the more efficient Modulation Coding and Scheme could be used to improve the spectrum efficiency. Further, in LTE release 8 and 9, single layer and dual layer beamforming on antenna port 5 and 7, 8 are already supported.
FIG. 1C schematically illustrates a flowchart of beamforming operation according to existing specifications. As illustrated in FIG. 1C, this operation mainly comprises beamforming weight and CQI acquirement process and beamforming and link adaptation process, which are illustrated by two big dashed blocks. As illustrated in the figure, at step S101, the UE transmits an uplink channel sounding reference signal (SRS) to the eNB. At step S102, the eNB obtains the channel state indication (CSI) information through the SRS information and calculates a beamforming weight based on the CSI information. At step S103, the UE obtains the CQI based on the CRS from the eNB and transmits the CQI to the eNB. At step S104, the eNB obtains the CQI. Then, at step S105, the eNB performs pre-coding and link adaptive operation based on the calculated beamforming weight and CQI indication. After that, at step S106, pre-coded data symbols and a UE specific reference signal (UE-RS) that is pre-coded in the same manner as those data symbols are transmitted to the UE. At step S107, after receiving the UE-RS, the UE performs demodulation on the received data symbols based on the received UE-RS.
The beamforming operation is based on non-codeword pre-coding and relies on the UE-RS for data demodulation. Because the UE-RS symbol is pre-coded with the same pre-coding matrix as the downlink data symbols, the UE can estimate out an effective channel. However, the UE-RS is transmitted only when the UE is being scheduled, and is therefore only transmitted over the frequency resource assignment of data transmission and can not be used as the resource for measuring the CQI by the UE. Therefore, it is based on the CRS assuming transmit diversity that the UE calculates the CQI, while the downlink data symbols are transmitted based on transmit beamforming. Therefore there is a CQI difference between the UE reported CQI and the actual CQI. Such CQI difference is actually a CQI difference between the transmit diversity and the transmit beamforming, or a CQI difference between CRS and UE-RS. Therefore, if eNB uses the UE reported CQI to transmit data, then the gain due to adopting beamforming will be lost, which directly causes degradation of the throughput performance. Therefore, there is a need for a scheme for performing CQI modification so as to mitigate the CQI difference.
To compensate for the CQI difference between the UE reported CQI and an actual CQI, an outer loop link adaptation scheme is disclosed by K. I. Pedersen, F. Frederiksen, T. E. Kolding, T. F. Lootsma, and P. E. Mogensen in an article entitled “Performance of high-speed downlink packet access in Coexistence with dedicated channels”, Trans. on VT, VOL. 56. NO. 3, May 2007. This scheme is a universal technical solution, that is to say, it can be used for either uplink or downlink. Hereinafter, a CQI measurement process using OLLA will be described with an OLLA for the downlink at UE side.
FIG. 2 illustrates a flowchart of a CQI measurement method performed in a CQI measurement unit (as illustrated in FIG. 1B) at the UE. As illustrated in FIG. 2, first at step S201, the CQI measurement unit 202 calculates SINR based on CRS received by a data receiving unit 201. Then, at step 202, OLLA operation is performed on SINR so as to perform CQI modification by adding an OLLA scaling factor A to the SINR, wherein detailed description regarding calculation of the scaling factor will be described hereinafter with reference to FIG. 3. Next, at step S203, the modified SINR is mapped to the CQI according to an SINR-CQI look-up table. The resulting CQI is transmitted to the eNB through a feedback transmitting unit 103.
FIG. 3 illustrates a flowchart of performing OLLA operation on the SINR. As illustrated in FIG. 3, first at step S301, a scaling factor A of the OLLA operation is initialized to be 0. Then, at step S302, codeword selection operation is performed based on the reported CQI, and then the method proceeds to step S303. If it is determined at step S303 that the codeword is successfully received, then at step S304, the scaling factor is increased by Aup dB; next, the method returns to step S302 to continue selection of codeword. If, at step S303, it is determined that the codeword is not successfully received, then at step S305, the scaling factor is decreased by Adown dB; next, the method returns to step S302 to continue selection of codeword. Further, it should be noted that such CQI adjustment may also be performed at eNB which may modify the OLLA scaling factor using an upturn factor Aup or a downturn factor Adown based on an ACK or NACK from the UE regarding whether the codeword is successfully received.
Besides, according to the OLLA scheme, the ratio of Adown to Aup is at least 9 in order to keep the block error probability (BLEP) within a predetermined threshold (in an actual application scenario, usually 0.1). In the actual application scenario, Aup=0.05, while Adown=0.45.
Therefore, OLLA is a simple but robust universal scheme. However, this modification is done by increasing or decreasing SINR/CQI based on an ACK/NACK feedback regarding codeword selection. Generally, it will take a long time to realize matching between a reported CQI and an actual CQI, especially when there is a bigger difference between an SINR estimated based on CRS and an actual SINR. Further, it is more susceptible to error. Once error occurs, it will significantly affect this modification and greatly prolong the time for modification. As a result, it will cause degradation of system performance.
Therefore, there is urgently needed an improved CQI modification scheme in the art.